1. Field of the Invention
The present invention generally relates to a paste composition and a solar cell employing the same, and more specifically, it relates to a paste composition employed for forming an electrically conductive layer such as an impurity layer or an electrode layer on a p-type silicon semiconductor substrate constituting a crystal silicon solar cell.
2. Description of the Background Art
A solar cell is known as an electronic component having electrodes formed on a p-type silicon semiconductor substrate. As shown in FIG. 1, such a solar cell is formed by a p-type silicon semiconductor substrate 1 having a thickness of 300 to 400 μm. An n-type impurity layer 2 having a thickness of 0.3 to 0.5 μm, an anti-reflective coating 3 and grid electrodes 4 are successively formed on a photoreceptive surface of the p-type silicon semiconductor substrate 1.
A back electrode layer 5 is formed on the back surface of the p-type silicon semiconductor substrate 1. This back electrode layer 5 is formed by applying a paste composition comprised of aluminum powder, glass frit and an organic vehicle by screen printing or the like, drying the same and thereafter firing the same at a temperature of at least 660° C. (the melting point of aluminum). In this firing, aluminum diffuses into the p-type silicon semiconductor substrate 1, thereby forming an Al—Si alloy layer 6 between the back electrode layer 5 and the p-type silicon semiconductor substrate 1 while simultaneously forming a p+ layer 7 as an impurity layer resulting from diffusion of aluminum atoms. The solar cell attains a BSF (back surface field) effect improving collection efficiency for formed carriers due to the presence of the p+ layer 7.
Another type of solar cell is put into practice by removing the back electrode layer 5 and the Al—Si alloy layer 6 with acid or the like while leaving the p+ layer 7 for ensuring the BSF effect and newly forming an electrode layer with silver paste or the like.
In order to reduce the cost for such a solar cell, means of reducing the thickness of the p-type silicon semiconductor substrate has recently been studied. When reduced in thickness, however, the p-type silicon semiconductor substrate is so deformed that the back surface formed with the back electrode layer by firing the paste composition is concaved due to the difference between the thermal expansion coefficients of silicon and aluminum, to result in warping. Therefore, cracking or the like is caused in steps of manufacturing the solar cell, to disadvantageously reduce the manufacturing yield of the solar cell.
In order to solve this problem, the quantity of the applied paste composition may be reduced to reduce the thickness of the back electrode layer. When the quantity of the applied paste composition is reduced, however, the quantity of aluminum diffusing into the p-type silicon semiconductor substrate from the surface thereof is insufficient. Consequently, no desired BSF effect can be attained and thus the characteristics of the solar cell are disadvantageously reduced.
In this regard, Japanese Patent Laying-Open No. 2000-90734, for example, discloses the composition of electrically conductive paste capable of ensuring desired characteristics of the solar cell and reducing the thickness of the back electrode layer. This electrically conductive paste contains an aluminum-containing organic compound in addition to aluminum powder, glass frit and an organic vehicle. However, the aforementioned prior art reduces the thickness of the back electrode layer thereby reducing the quantity of warping caused in the p-type silicon semiconductor substrate. Under the present circumstances, there is developed neither method nor composition of electrically conductive paste for reducing the quantity of warping without reducing the quantity of the applied electrically conductive paste for sufficiently attaining a desired BSF effect.